Membrane and use thereof

ABSTRACT

The present invention relates to a solid multicomponent membrane for use in a reactor where the membrane comprises a mixed metal oxide having a structure represented by the formula: La 1−x Ca x (Fe 1−y−y′ Ti y Al y′ ) w O 3−4  wherein x, y, y′, w, and d each represent a number such that 0.1≦(y+y′)≦0.8, 0.15≦(x+y′) ≦0.95, 0.05≦(x−y)≦0.3, 0.95&lt;w&lt;1, and d equals a number that renders the compound charge neutral and is not less than zero and not greater than about 0.8. Furthermore, the present invention relates to a use of the membrane in a reactor for generating heat or for generating synthesis gas.

The present invention relates to a solid multicomponent membrane whichis particularly suited as dense oxygen separation membrane inapplications with high driving forces for oxygen transport.

Inorganic membranes show promise for use in commercial processes forseparating oxygen from an oxygen containing gaseous mixture. Envisionedapplications range from small scale oxygen pumps for medical use tolarge scale integrated gasification combined cycle plants. Thistechnology encompasses two different kinds of membrane materials; oxygenion conductors and mixed oxygen ion and electronic conductors. In bothcases the oxygen ion transport is by oxygen ion vacancies orinterstitial oxygen in the membrane material. In the case of mixedconductors electrons are also transported in the membrane material.

Membranes formed from mixed conducting oxides can be used to selectivelyseparate oxygen from an oxygen containing gaseous mixture at elevatedtemperatures. Oxygen transport occurs when a difference in the chemicalpotential of oxygen (Δlogp_(O2)) exists across the membrane. On the highoxygen partial pressure side of the membrane, molecular oxygendissociates into oxygen ions which migrate to the low oxygen partialpressure side of the membrane and recombine there to form oxygenmolecules. Electrons migrate through the membrane in the oppositedirection to conserve charge. The rate at which oxygen permeates throughthe membrane is mainly controlled by three processes; (I) the rate ofoxygen exchange at the high oxygen partial pressure surface of themembrane, (II) the oxygen diffusion rate within the membrane and (III)the rate of oxygen exchange on the low oxygen partial pressure surfaceof the membrane. If the rate of oxygen permeation is controlled by theoxygen diffusion rate, the oxygen permeability is known to be inverselyproportional to the membrane thickness (Fick's law). If the membranethickness is decreased below a certain critical membrane thickness whichdepends on temperature and other process parameters, surface oxygenexchange on one or both membrane surfaces will become oxygen permeationrate limiting. The rate of oxygen permeation is then independent of themembrane thickness.

During recent years the use of dense mixed conducting membranes invarious processes has been described. Examples are oxygen productiondescribed in European Patent Application No. 95100243.5 (EP-A-663230),U.S. Pat. No. 5,240,480, U.S. Pat. No. 5,447,555, U.S. Pat. No.5,516,359 and U.S. Pat. No. 5,108,465, partial oxidation of hydrocarbonsdescribed in U.S. Pat. No. 5,714,091 and European Patent Application No.90134083.8 (EP-A438902), production of synthesis gas described in U.S.Pat. No. 5,356,728 and enrichment of a sweep gas for fossil energyconversion with economical CO₂ abatement as described in PCT/NO97/00170,PCT/NO97/00171 and PCT/NO97/00172.

For the application of MCM (Mixed Conducting Membrane) technology, themembrane material must fulfil certain requirements in addition to beinga good mixed conductor. These fall into three categories; thermodynamicstability under static conditions, thermodynamic stability under dynamicconditions, and mechanical stability. The membrane material must bethermodynamically stable under any static condition within theappropriate temperature and oxygen partial pressure range. Furthermore,the membrane material must be stable against reaction with theadditional components in the gaseous phase (e.g. CO₂, H₂O, NO_(x),SO_(x)), and any solid phase in contact with it (e.g., seals and supportmaterial). This calls for different materials for differentapplications.

A membrane material that fulfils all the stability requirements understatic conditions, may still be unstable when it is placed in apotential gradient. Any multi-component material kept in a potentialgradient, e.g. oxygen partial pressure gradient or electrical potentialgradient, will be subjected to driving forces acting to demix ordecompose the material. These phenomena are called kinetic demixing andkinetic decomposition and are well described in the literature (e.g.,Schmalzried, H. and Laqua, W., Oxidation of Metals 15 (1981) 339).

Kinetic demixing acts to gradually change the cationic composition ofthe membrane along the axis parallel to the applied potential. Thisphenomenon will always occur in materials where a mixture of cations arepresent on the same sublattice. Kinetic demixing may or may not reducethe performance and lifetime of the membrane.

Kinetic decomposition implies a total breakdown of the compound orcompounds comprising the membrane, and results in the appearance ofdecomposition compounds on the membrane surface. This phenomenon occursin all multicomponent materials when placed in a potential gradientexceeding a certain critical magnitude. A membrane kept in an oxygenpartial pressure gradient large enough for kinetic decomposition to takeplace, will have its performance and lifetime reduced. Those skilled inthe art recognize the phenomenon of kinetic decomposition as one of themajor critical parameters in developing durable membranes, particularlyfor processes involving large potential gradients across the membrane.

Furthermore, when the membrane is placed in an oxygen chemical potentialgradient and it responds by establishing a gradient in the concentrationof oxygen vacancies or interstitials parallel to the direction of theapplied potential, the membrane experiences mechanical stress with thestrain plane perpendicular to the direction of the applied potentialgradient. This mechanical stress is caused by a phenomenon referred toas chemical expansion, which can be defined as the dependency of theunit cell volume of the nonstoichiometric oxide on the oxygenstoichiometry. When the chemical expansion exceeds a critical limit, andgives rise to mechanical stress exceeding a critical limit governed bythe membrane package design, a mechanical failure of the membranepackage may result Those skilled in the art recognize the phenomenon ofchemical expansion as one of the major critical parameters in developingdurable membrane packages.

Two prior art processes can be put forward as particularly relevant tothe present invention: the production of synthesis gas in which anoxygen containing gas is fed to the first side of a membrane, wherebypure oxygen is transported through the membrane, and the so producedoxygen partially oxidizes a hydrocarbon containing gas supplied to thesecond side of the membrane; and fossil energy conversion witheconomical CO₂ abatement (e.g. PCT/NO97/00172) where an oxygencontaining gas is fed to the first side of a membrane, whereby pureoxygen is transported through the membrane, and the produced oxygenoxidizes a hydrocarbon containing gas supplied to the second side of themembrane.

The process conditions of the relevant process define the environs ofthe membrane and play a determining role in the selection of membranematerial. Examples of typical process parameters for the two saidprocesses are given in Tables 1 and 2, respectively. Both processes arecharacterized by a logp_(O2) gradient across the membrane of well above10 decades. Furthermore, both processes call for membrane materials thathave a high stability against reaction with CO₂ under reducingconditions, as the CO₂ pressure is well above 1 bar.

TABLE 1 Example of process parameters for an MCM syngas productionprocess Fuel side Air side Temperature 750-950° C. 750-950° C. Totalpressure 30 bar 1.5 bar p_(O2) 10⁻¹⁷ bar 0.03-0.23 bar p_(CO2) 3-5 bar0.04-0.05 bar Other major components H₂, CO, H₂O, CH₄ N₂

TABLE 2 Example of process parameters for an MCM power productionprocess Fuel side Air side Temperature 1100-1200° C. 1100-1200° C. Totalpressure 12-32 bar 10-30 bar p_(O2) appr. 10⁻¹² bar 0.5-5 bar p_(CO2)0-12 bar <2 bar Other major components H₂O, CH₄ N₂

During recent years dense mixed conducting membranes have beendescribed.

U.S. Pat. No. 5,306,411 discloses a solid, gas-impervious,electron-conductive, oxygen ion-conductive, single-phase membrane foruse in an electrochemical reactor, said membrane being formed from aperovskite represented by the formula:

A_(s)A′_(t)B_(u)B′_(v)B″_(w)O_(x)

wherein A represents a lanthanide, Y, or mixture thereof; A′ representsan alkaline earth metal or mixture thereof; B represents Fe; B′represents Cr, Ti, or mixture thereof; and B″ represents Mn, Co, V, Ni,Cu, or mixture thereof and s, t, u, v, w, and x each represent a numbersuch that:

s/t equals from about 0.01 to about 100;

u equals from about 0.01 to about 1;

v equals from about 0.01 to 1;

w equals from zero to about 1;

x equals a number that satisfies the valences of the A, A′, B, B′ and B″in the formula; and

0.9<(s+t)/(u+v+w)<1.1.

The examples focusing on A′ representing Sr and B′ representing Cr.

U.S. Pat. No. 5,712,220 describes compositions capable of operatingunder high carbon dioxide partial pressures for use in solid-stateoxygen producing devices represented by the formulaLn_(x)A′_(x)A″_(x)B_(y)B′_(y)B″_(y)O_(3−x), wherein Ln is an elementselected from the f block lanthanides, A′ is selected from Group 2, A″is selected from Groups 1, 2 and 3 and the f block lanthanides, and B,B′, B″ are independently selected from the d block transition metals,excluding titanium and chromium, wherein 0<=x<1, 0<x′<1, 0<=x″<1,0<y<1.1, 0<y′<1.1, 0<=y″<1.1, x+x′+x″=1.0, 1.1>y+y′+y″>1.0 and z is anumber which renders the compound charge neutral wherein such elementsare represented according to the Periodic Table of the Elements adoptedby IUPAC. The examples focusing on A′ representing Sr or Ba, Brepresenting Co, B′ representing Fe, and B″ representing Cu.

WO97/41060 describes a solid state membrane for use in a catalyticmembrane reactor wherein said membrane is fabricated from a mixed metaloxide material having a brownmillerite structure and having the generalstoichiometry A_(2−x)A′_(x)B_(2−y)B′_(y)O_(5+z), where A is an alkalineearth metal ion or mixture of alkaline earth metal ions; A′ is a metalion or mixture of metal ions where the metal is selected from the groupconsisting of the lanthanide series or is yttrium; B is a metal ion ormixture of metal ions wherein the metal is selected from the groupconsisting of 3d transition metals, and the group 13 metals; B′ is ametal ion or mixture of metal ions where the metal is selected from thegroup consisting of the 3d transition metals, the group 13 metals, thelanthanides and yttrium; x is a number greater than 0 and less than 2, yis a number greater than 0 and less than or equal to 2, and z is anumber greater than zero and less than one that renders the compoundcharge neutral. The examples focus on the most preferred combination ofelements given by A representing Sr, A′ representing La, B representingGa, and B′ representing Fe.

U.S. Pat. No. 5,306,411, U.S. Pat. No. 5,712,220, and WO97/41060 eachencompass wide ranges of membrane compositions. It is known to thoseskilled in the art that a great number of compositions encompassed bythe claims of U.S. Pat. No. 5,306,411 and U.S. Pat. No. 5,712,220 areinherently unstable as perovskites and that a great number ofcompositions encompassed by WO97/41060 are inherently unstable asbrownmillerites under all conditions relevant to membrane processes.Furthermore, a large number of the compositions encompassed by U.S. Pat.No. 5,306,411, U.S. Pat. No. 5,712,220, and WO97/41060 are characterisedby low or zero oxygen flux under all conditions relevant to membraneprocesses.

The main object of the present invention was to arrive at an improvedmembrane showing good stability against reaction with carbon dioxide andagainst reduction of oxide components to metal.

Another object of the present invention was to arrive at an improvedmembrane showing stability against kinetic decomposition and resistanceto mechanical failure due to chemical expansion stresses.

The inventors found that a certain class of multicomponent metallicoxides are particularly suited as membrane materials in processes inwhich the membrane is subjected to a large potential gradient, e.g.oxygen partial pressure difference of 6-7 orders of magnitude or moreacross the membrane. These compositions overcome problems associatedwith kinetic decomposition. Additionally, due to their low chemicalexpansion and high stability against carbon dioxide and water, thesematerials are particularly suited as membranes for the production ofsyngas and for fossil energy conversion with economical CO₂ abatement.

The compositions according to the present invention are based on the socalled perovskite structure, named after the mineral perovskite, CaTiO₃,but the cation stoichiometry is different from the ideal perovskite, andit is this difference that gives the compositions according to thepresent invention superior stability in a potential gradient.Furthermore, the process conditions associated with the production ofsyngas or fossil energy conversion with economical CO₂ abatement limitthe selection of elements of which the perovskite membrane can consist.

A material possessing the perovskite structure can in its most generalform be written A_(v)B_(w)O_(3−d), where A and B each represent anycombination and number of elements provided that the ionic radii of theelements, as defined and tabulated by Shannon (Acta Cryst. (1976) A32,751), satisfy the requirement that the number t defined by$t = \frac{r_{A} + r_{O}}{\sqrt{2}\left( {r_{B} + r_{O}} \right)}$

is not less than about 0.85 and not greater than about 1.10, andpreferably t is not less than about 0.95 and not greater than about1.05, where r_(A) and r_(B) represent the weighted average ionic radiusof the A-elements and the B-elements, respectively, r_(O) represents theionic radius of the oxygen ion; and v, w, and d each represent numberssuch that 0.9<v<1.05, 0.9<w<1.05, and d is not less than zero and notgreater than about 0.8, and preferably 0.95<v<1.03 and 0.95<w<1.03.

The perovskite membrane for use in said processes must contain at leastone element (I) whose valence is substantially mixed under said processconditions, and (II) with the additional requirement that the oxide ofsaid element, or of any additional element of which the membrane iscomposed, does not reduce to a metal under any condition encompassed bysaid process conditions. This requirement points to the group of 3dtransition metals, but with the limitation expressed by part (I) of therequirement excluding Sc, Ti, V, Cr, and Zn as the mixed valenceelement, and part (II) excluding Co, Ni, and Cu. Therefore, only Fe andMn satisfy part (I) and part (II) of said requirement, and, hence, themembrane must contain Fe or Mn or mixture thereof. The membrane can notcontain Co, Ni, or Cu. Therefore, the preferred compositions of U.S.Pat. No. 5,712,220, referenced in the “Background of the invention”, cannot be used as membranes in said processes. Said preferred compositionsof U.S. Pat. No. 5,712,220 are expected to decompose under theconditions fo the said two processes, resulting in decreasingly pooroxygen permeation and eventually to cracking and complete breakdown ofthe membrane.

Said perovskite membrane containing Fe or Mn or a mixture thereof as theB cation(s) or as constituents of the mixture of B cations, must containA cations stable as di- or tri-valent oxides of suitable ionic radiirelative to the ionic radii of Fe and Mn according to said requirementfor the value of t. This limitation in combination with the exclusion ofradioactive elements effectively excludes all elements according to thePeriodic Table of the Elements adopted by IUPAC, except Ca, Sr, Ba, andLa.

Among the oxides of Ca, Sr, Ba, and La, the oxides of Sr and Ba are notsufficiently stable with respect to formation of carbonates, SrCO₃ andBaCO₃, to be used in said processes for which typical process parameterswere given in Tables 1 and 2. The stability of the oxides of Ca, Sr, Ba,and La relative to the corresponding carbonates are shown in FIG. 1.Hence, for said processes, only La and Ca can be used as A-cations inthe perovskite of which the membrane consists. The exclusion of Sr andBa as constituents of the membrane, excludes the use of the preferredcompositions of U.S. Pat. No. 5,712,220, U.S. Pat. No. 5,306,411, andWO97/41060, referenced in the “Background of the invention”, asmembranes in said processes. Said preferred compositions of U.S. Pat.No. 5,712,220, U.S. Pat. No. 5,306,411, and WO97/41060, all containingSr or Ba, are expected to react with CO₂ and decompose under theformation of SrCO₃ and BaCO₃ under the conditions of said two processes,resulting in decreasingly poor oxygen permeation and eventually crackingand complete breakdown of the membrane.

In addition to containing the mixed valence elements Fe or Mn, or amixture of Mn and Fe, the perovskite for use as a membrane in saidprocesses can also contain one or more fixed valence elements as Bcations; fixed valence meaning here that the particular ion hassubstantially the same valency at any spatial point in the membrane andat any time for the relevant process. The presence of such fixed valenceelements may be needed in order to Increase the stability of theperovskite, to decrease the chemical expansion, to prevent ordering, orto enhance the performance of the perovskite as a membrane material inany other manner. The ions of the fixed valence elements must be ofsuitable ionic radii relative to the other B cations and A cations,according to said requirement for the value of t defined above. Thislimitation excludes all other elements than Ti, Cr, Al, Ga, Ge and Be.Furthermore, due to high vapor pressures of Ge containing species andlow melting temperatures, Ge has to be excluded. Be is excluded ongrounds of toxicity and high vapor pressure of the hydrate of beryllium.Of the remaining elements Al and Ga are expected to have similar effectas constituents in the perovskite, but the ionic radius of the Al ion ismore favorable than of the Ga ion. Furthermore, the cost of Al isconsiderably lower than the cost of Ga. Hence, Ga can be excluded on thegrounds of Al being a better choice. Under oxidizing conditions, thevapor pressure of CrO₃(g) above chromium containing perovskites is high,and Cr is preferably avoided. Therefore, as a fixed valence B cation,only Ti and Al will be considered further.

The exclusion of Ga and Cr excludes the use of the preferredcompositions of U.S. Pat. No. 5,306,411 and WO97/41060, referenced inthe “Background of the invention”, as membranes in said processes. Saidpreferred compositions of U.S. Pat. No. 5,306,411 containing Cr, areexpected to become depleted in Cr as CrO₃(g) evaporates from the surfaceof the membrane under the conditions of said two processes, resulting indecomposition of the membrane material and the formation of newcompounds, which yields decreasingly poor oxygen permeation andeventually cracking and complete breakdown of the membrane.

According to said requirements, limitations, and exclusions treatedabove, the membrane material possessing the perovskite structure for usein said processes, must have a composition represented by the formula:

(La_(1−x)Ca_(x))_(v)(B_(1−y)B′_(y))_(w)O_(3−d)

wherein B represents Fe or Mn or mixture thereof; B′ represents Ti or Alor mixture thereof; and x, y, v, w, and d each represent a number suchthat 0≦x≦1, 0≦y≦1, 0.9≦v≦1, 0.9≦w≦1, and d equals a number that rendersthe compound charge neutral and is not less than zero and not greaterthan about 0.8, and preferably 0.95≦v≦1 and 0.95≦w≦1.

Compositions containing no Ti or Al, i.e. y=0, are characterized by toohigh chemical expansion, as exemplified by the present Example 18, andcan not be used as membranes in said processes. The chemical expansionis higher for compositions containing Mn than for compositionscontaining Fe.

Compositions containing Ti, Al, or Ti and Al, i.e. B′ represents Ti, Al,or a mixture of Ti and Al, and y>0, are characterized by an improved(lower) chemical expansion as compared with compositions containing noTi and no Al, i.e. y=0, as exemplified by a comparison of the presentexamples 17 and 18. The compositions of Example 17 with B representingFe display chemical expansion characteristics that are acceptable for amembrane material in said processes.

Compositions containing Ti and Al, i.e. B′ represents a mixture of Tiand Al, and y>0, are characterized by a further improvement (reducton)in the chemical expansion compared with compositions where B′ representsTi and y>0, as exemplified by a comparison of the present Examples 17and 21. The composition of the present Example 21 with B representing Fedisplays excellent chemical expansion characteristics for a membranematerial in said processes.

Although compositions containing Mn and Ti, Al, or Ti and Al, i.e. B′represents Ti, Al, or mixture of Ti and Al, and y>0 and B represents Mn,are characterized by an improved (lower) chemical expansion as comparedwith compositions containing no Ti or Al, i.e. y=0, the improvement isnot large enough to render these compositions acceptable as membranematerials in said processes. The membrane can, therefore, not containsubstantial amounts of Mn. The present Example 20 exemplifies the highchemical expansion of Mn containing materials.

Following the discussion and further limitations hitherto, the membranematerial possessing the perovskite structure for use in said processes,must have a composition represented by the formula:

(La_(1−x)Ca_(x))_(v)(Fe_(1−y−y′)Ti_(y)A_(y′))_(w)O_(3−d)

wherein x, y, y′, v, w, and d each represent a number such that 0≦x≦1,0≦y≦1, 0≦y′<1, 0<(y+y′)<1, 0.9≦v≦1, 0.9≦w≦1, and d equals a number thatrenders the compound charge neutral and is not less than zero and notgreater than about 0.8, and preferably 0.95≦v≦1 and 0.95≦w≦1.

Stoichiometric perovskite compositions represented by said formula, i.e.v=w=1, are kinetically unstable when subjected to large gradients (6-7orders of magnitude or more) in the oxygen partial pressure. The kineticdecomposition that occurs in these materials gives rise to the formationof decomposition products on at least one of the membrane surfaces and adecrease in the oxygen flux with time. Such kinetic decomposition in thestoichiometric perovskite materials is exemplified by the presentexamples 12 and 15 and FIGS. 4, 8, 9, and 10. Kinetic decompositionbecomes more pronounced when w>v. Therefore, stoichiometric perovskites(v=w), or perovskites with A-site deficiency (w>y) represented by saidformula can not be used as membranes in said processes.

The exclusion of stoichiometric and A-site deficient perovskites,excludes the use of the compositions of U.S. Pat. No. 5,712,220 andWO97/41060, and excludes the use of the preferred compositions of U.S.Pat. No. 5,306,411 referenced in the “Background of the invention”, asmembranes in said processes. Said compositions of U.S. Pat. No.5,712,220, U.S. Pat. No. 5,306,411, and WO97/41060, are expected todecompose in the large oxygen partial pressure gradient of said twoprocesses, resulting in decreasingly poor oxygen permeation andeventually to cracking and complete breakdown of the membrane.

Compositions represented by said formula, and where the numbers v and ware selected such that v=1 and 0.95≦w<1, however, are stable withrespect to kinetic decomposition even in oxygen partial pressuregradients of well above 10 decades. Under certain additionalrequirements regarding the values of x and y of the enumerated formula,said compositions are characterized by stable oxygen flux not decreasingwith time, and single phase unchanged membrane surfaces and interior.Examples of the performance of such compositions are presented in thepresent Examples 11, 13, and 14 and FIGS. 3, 5, 6, and 7.

Following the further limitations pointed out in the discussionhitherto, the membrane material possessing the perovskite structure foruse in said processes, must have a composition represented by theformula

La_(1−x)Ca_(x)(Fe_(1−y−y′)Ti_(y)Al_(y′))_(w)O_(3−d)

wherein x, y, y′, w, and d each represent a number such that 0≦x≦1,0≦y≦1, 0≦y′<1, 0<(y+y′)<1, y≦x, 0.95<w<1, and d equals a number thatrenders the compound charge neutral and is not less than zero and notgreater than about 0.8.

The compositions represented by said formula can alternatively berepresented by mixtures of y number of moles of CaTi_(w)O_(3−d′)(CT),(x−y) number of moles of CaFe_(w)O_(3−d″) (CF), (1−x−y) number of molesof LaFe_(w)O_(3−d′″) (LF), and y′ number of moles of LaAl._(w)O_(3−d″″)(LA), with respective mole fractions given by X_(CT)=y, X_(CF)=x−y,X_(LF)=1−x−y′, and X_(LA)=y′. Graphically, said mixtures can berepresented within a ternary phase diagram as shown in the present FIG.14.

Compositions represented by said formula, and where the numbers x, y,and y′ are selected such that (y+y′)<0.1 and (x−y)≦0.3 are characterizedby having high chemical expansion, and membranes of these compositionscan probably not be used in said processes. Examples of the highchemical expansion of these materials are presented in the presentExamples 18 and 19.

Compositions represented by said formula, and where the numbers x and yare selected such that (x−y)<0.05 are characterized by having lowvacancy concentrations (d), which yield low oxygen flux rates, andmembranes of these compositions can probably not be used in saidprocesses. An example of the low oxygen flux of these compositions isprovided in the present Example 23.

Compositions represented by said formula, and where the numbers x, y,and y′ are selected such that either (y+y′)>0.8, or (1−x−y′)<0.05 and(x−y)≦0.3, are characterized by having low electronic conductivity,which yield low oxygen flux rates, and membranes of these compositionscan probably not be used in said processes. An example of the low oxygenflux of these compositions is provided in the present Example 24.

Compositions represented by said formula, and where the numbers x and yare selected such that (x−y)>0.3, are not simple perovskites atconditions representative of said processes. The cations and oxygenvacancies of these compositions become ordered, during which orderingprocess the flux rates decrease to eventually reach too low permeationrates to be used as membranes in said processes. An example of the lowoxygen flux of these compositions is provided in the present Example 25.

Compositions represented by said formula, and where the numbers x and yare selected such that 0.1≦(y+y′)≦0.8, 0.15≦(x+y′)≦0.95, and0.05≦(x−y)≦0.3 are characterized by having properties acceptable for useas membranes in said processes. These properties include low andacceptable chemical expansion below 0.1% (Examples 17 and 21),sufficiently high vacancy concentration to yield sufficient flux rates(Example 11), sufficiently high electronic conductivity to yieldsufficient flux rates (Example 11), minor (acceptable) or no ordering ofcations and oxygen vacancies (Examples 11, 13 and 14).

Compositions represented by said formula and where the numbers x and yare selected such that 0.1≦(y+y′)≦0.8, 0.15≦(x+y′)≦0.95, 0.05≦(x−y)≦0.3,and y′>0, are characterized by a further reduction in the chemicalexpansion (Example 21).

Thus, the membrane material according to the present invention for usein said processes has a composition represented by the formula:

La_(1−x)Ca_(x)(Fe_(1−y−y′)Ti_(y)Al_(y′))_(w)O_(3−d)

wherein x, y, y′, w, and d each represent a number such that0.1≦(y+y′)≦0.8, 0.15≦(x+y′)≦0.95, 0.05≦(x−y)≦0.3, 0.95<w<1, and d equalsa number that renders the compound charge neutral and is not less thanzero and not greater than about 0.8.

Particularly suitable compositions according to the present inventionare represented by said general formula wherein x, y, y′, w, and d eachrepresent a number such that 0.15<(y+y′)<0.75, 0.2<(x+y′)<0.9,0.05<(x−y)<0.15, 0.95<w<1, and d equals a number that renders thecompound charge neutral and is not less than zero and not greater thanabout 0.8.

Representative compositions includeLa_(0.65)Ca_(0.35)Fe_(0.63)Ti_(0.24)Al_(0.10)O_(3−d),La_(0.45)Ca_(0.55)Fe_(0.48)Ti_(0.39)Al_(0.10)O_(3−d),La_(0.4)Ca_(0.6)Fe_(0.49)Ti_(0.43)Al_(0.05)O_(3−d),La_(0.58)Ca_(0.42)Fe_(0.63)Ti_(0.31)Al_(0.03)O_(3−d),La_(0.4)Ca_(0.6)Fe_(0.485)Ti_(0.485)O_(3−d),La_(0.55)Ca_(0.45)Fe_(0.63)Ti_(0.34)O_(3−d),La_(0.68)Ca_(0.32)Fe_(0.73)Ti_(0.25)O_(3−d) andLa_(0.22)Ca_(0.78)Fe_(0.34)Ti_(0.62)O_(3−d).

The improvements afforded by the applicants' invention can be bestappreciated by a comparison of properties, such as structure,performance during oxygen permeation, phase composition after permeationetc., of the claimed non-stoichiometric compositions with the prior artstoichiometric compositions.

The invention will be further explained and envisaged in the examplesand the figures.

FIG. 1 shows the upper stability limit of selected oxides againstreaction with carbon dioxide as a function of temperature.

FIG. 2 shows X-ray diffractograms of the B-site deficient membranematerial of Example 1 and the cation stoichiometric membrane material ofExample 2.

FIG. 3 shows oxygen permeation characteristics of the membrane materialof Example 1.

FIG. 4 shows oxygen permeation characteristics of the membrane materialof Example 2.

FIG. 5 shows X-ray diffractograms of the membrane material of Example 1before and after the oxygen permeation experiment of Example 11.

FIG. 6 shows a scanning electron micrograph of the high oxygen partialpressure (primary) side of the membrane material of Example 4 after anoxygen permeation experiment.

FIG. 7 shows a scanning electron micrograph of the low oxygen partialpressure (secondary) side of the membrane material of Example 4 after anoxygen permeation experiment.

FIG. 8 shows X-ray diffractograms of the membrane material of Example 2before and after the oxygen permeation experiment of Example 12.

FIG. 9 shows a scanning electron micrograph of the high oxygen partialpressure (primary) side of the membrane material of Example 2 after theoxygen permeation experiment of Example 12.

FIG. 10 shows scanning electron micrograph of the low oxygen partialpressure (secondary) side of the membrane material of Example 2 afterthe oxygen permeaton experiment of Example 12.

FIG. 11 shows X-ray diffractograms of the membrane material of Example 5before and after an oxygen permeation experiment.

FIG. 12 shows a scanning electron micrograph of the high oxygen partialpressure (primary) side of the membrane material of Example 5 after anoxygen permeation experiment.

FIG. 13 shows a scanning electron micrograph of the low oxygen partialpressure (secondary) side of the membrane material of Example 5 after anoxygen permeation experiment.

FIG. 14 shows the range of the claimed compositions represented in aternary diagram as mixtures of LaFe_(1−v)O₃ (LF), CaTi_(1−v)O₃ (CT),LaAl_(1−v)O₃ (LA) and CaFe_(1−v)O_(2.5) (CF).

EXAMPLE 1 Preparation of La_(0.4)Ca_(0.6)Fe_(0.485)Ti_(0.485)O_(3−d)

A solid mixed conducting membrane was prepared by a soft chemistry routewherein the appropriate amounts of La₂O₃, CaCO₃, and titanylacetylacetonate were first dissolved in nitric acid. To this liquidmixture was added the appropriate amount of a preprepared standardized1M aqueous solution of Fe(NO₃)₃. To the mixture was added citric acid inexcess, and excess water was evaporated for 3 hours at 90° C., duringwhich time complexation takes place. The resulting gel was pyrolyzed inair for 14 hours by heating to 140° C., whereupon the resulting drypowder was calcined at 500° C. for 2 hours and 900° C. for 10 hours. Thepowder mixture was then combined with a binder and uniaxially coldpressed to a 13 mmø disk at 180 MPa. The resulting porous disk washeated to 500° C. at 5°/min to allow controlled combustion of thebinder, and then further heated to 1250° C., maintained at 1250° C. for3 hours and cooled to room temperature. This procedure yielded a 10 mmøgas tight disk with>96% of theoretical density. The membrane waspolished on both sides to a 1 micron surface finish and 1.66 mmthickness. The formula representing the product may be expressed asLa_(0.4)Ca_(0.6)Fe_(0.485)Ti_(0.485)O_(3−d).

EXAMPLE 2 (COMPARATIVE) Preparation ofLa_(0.4)Ca_(0.6)Fe_(0.5)Ti_(0.5)O_(3−d)

A solid mixed conducting membrane was prepared according to the methodof Example 1 except the amounts of the reactants were chosen to yield aproduct that may be represented by the formulaLa_(0.4)Ca_(0.6)Fe_(0.5)Ti_(0.5)O_(3−d). The membrane was polished onboth sides to a 1 micron surface finish and 1.00 mm thickness.

EXAMPLE 3 (COMPARATIVE) Preparation ofLa_(0.2)Sr_(0.8)Fe_(0.8)Cr_(0.1)Co_(0.1)O_(3−d)

A solid mixed conducting membrane was prepared by a soft chemistry routewherein the appropriate amounts of La₂O₃ and Sr(NO₃)₂ were firstdissolved in nitric acid. To this liquid mixture was added theappropriate amounts of preprepared standardized 1M aqueous solutions ofFe(NO₃)₃, Cr(NO₃)₃, and Co(NO₃)₂. To the mixture was added citric acidin excess, and excess water was evaporated for 3 hours at 90° C., duringwhich time complexation takes place. The resulting gel was pyrolyzed inair for 14 hours by heating to 140° C., whereupon the resulting drypowder was calcined at 500° C. for 2 hours and 900° C. for 10 hours. Thepowder mixture was then combined with a binder and uniaxially coldpressed to a 13 mmø disk at 180 MPa. The resulting porous disk washeated to 500° C. at 5°/min to allow controlled combustion of thebinder, and then further heated to 1200° C., maintained at 1200° C. for3 hours and cooled to room temperature. This procedure yielded a 10 mmøgas tight disk with >96% of theoretical density. The membrane waspolished on both sides to a 1 micron surface finish and 1.5 mmthickness. The formula representing the product may be expressed asLa_(0.2)Sr_(0.8)Fe_(0.8)Cr_(0.1)C_(0.1).O_(3−d).

EXAMPLE 4 Preparation of La_(0.55)Ca_(0.45)Fe_(0.63)Ti_(0.34)O_(3−d)

A solid mixed conducting membrane was prepared according to the methodof Example 1 except the amounts of the reactants were chosen to yield aproduct that may be represented by the formulaLa_(0.55)Ca_(0.45)Fe_(0.63)Ti_(0.34)O_(3−d). The membrane was polishedon both sides to a 1 micron surface finish and 1.42 mm thickness.

EXAMPLE 5 Preparation of La_(0.3)Ca_(0.7)Fe_(0.485)Ti_(0.485)O_(3−d)

A solid mixed conducting membrane was prepared according to the methodof Example 1 except the amounts of the reactants were chosen to yield aproduct that may be represented by the formulaLa_(0.3)Ca_(0.7)Fe_(0.485)Ti_(0.485)O_(3−d). The membrane was polishedon both sides to a 1 micron surface finish and 1.46 mm thickness.

EXAMPLE 6 (COMPARATIVE) Preparation of La_(0.8)Ca_(0.2)FeO_(3−d)

A solid mixed conducting membrane was prepared according to the methodof Example 1 except titanium acetylacetonate was omitted and the amountsof the other reactants were chosen to yield a product that may berepresented by the formula La_(0.8)Ca_(0.2)FeO_(3−d). The membrane wasground to a square of approximately 8×8 mm and polished on both sides toa 1 micron surface finish.

EXAMPLE 7 (COMPARATIVE) Preparation ofLa_(0.6)Ca_(0.4)Fe_(0.777)Ti_(0.194)O_(3−d)

A solid mixed conducting membrane was prepared according to the methodof Example 1 except the amounts of the reactants were chosen to yield aproduct that may be represented by the formulaLa_(0.6)Ca_(0.4)Fe_(0.777)Ti_(0.194)O_(3−d). The membrane was ground toa square of approximately 8×8 mm and polished on both sides to a 1micron surface finish.

EXAMPLE 8 (COMPARATIVE) Preparation ofLa_(0.2)Ca_(0.8)Mn_(0.4)Ti_(0.6)O_(3−d)

A solid mixed conducting membrane was prepared according to the methodof Example 1 except manganese nitrate solution was substituted for ironnitrate solution and the amounts of the reactants were chosen to yield aproduct that may be represented by the formulaLa_(0.2)Ca_(0.8)Mn_(0.4)Ti_(0.6)O_(3−d). The membrane was ground to asquare of approximately 8×8 mm and polished on both sides to a 1 micronsurface finish.

EXAMPLE 9 Preparation ofLa_(0.65)Ca_(0.35)Fe_(0.63)Ti_(0.24)Al_(0.10)O_(3−d)

A solid mixed conducting membrane was prepared according to the methodof Example 1 except aluminium acetylacetonate was added in addition tothe other components and the amounts of the reactants were chosen toyield a product that may be represented by the formulaLa_(0.65)Ca_(0.35)Fe_(0.63)Ti_(0.24)Al_(0.10)O_(3−d). The membrane waspolished on both sides to a 1 micron surface finish and 1.5 mmthickness.

EXAMPLE 10 Structure of La_(0.4)Ca_(0.6)Fe_(0.485)Ti_(0.485)O_(3−d) andLa_(0.4)Ca_(0.6)Fe_(0.5)Ti_(0.5)O_(3−d)

XRD diffractograms of the mixed conducting membrane materials ofExamples 1 (“B-site deficient”) and 2 (“Stoichiometric”) are shown inFIG. 2. Both materials are single phase and possess the perovskitestructure. Peaks marked “Si” in the diffractogram of the B-sitedeficient material belong to silicon, which was added as an internalXRD-standard. A slight shift of the peak locations to lower diffractionangles in the B-site deficient material shows that the unit cell volumeis increased by introduction of B-site deficiency.

EXAMPLE 11 Oxygen Permeation Test of a Dense Mixed ConductingLa_(0.4)Ca_(0.6)Fe_(0.485)Ti_(0.485)O_(3−d) Membrane

The mixed conducting membrane disk of Example 1 was attached to analumina tube by placing one gold ring between the membrane and thealumina tube and one gold ring between the membrane and a quartz supportstructure. The membrane assembly was heated to 1031° C. where the goldsoftened and a seal formed between the membrane and the alumina tube.250 ml/min (STP) of a mixture of 50% oxygen and 50% nitrogen was flushedacross the outside (high p_(O2) or primary) surface of the membrane.

In the first part of the test, 250 ml/min (STP) of He was flushed acrossthe inside (low p_(O2) or secondary) surface of the membrane. Oxygenpermeated through the membrane from the high p_(O2) side to the lowp_(O2) side and was entrained by the He sweep gas stream. The oxygenconcentration in the exiting helium stream was analyzed by gaschromatography. Small leakages due to imperfections in the gold ringseal were detected by analyzing the exiting helium stream for nitrogen.The oxygen flux was calculated by the following formula:$J_{O_{2}} = {\left( {X_{O_{2}} - X_{N_{2}}} \right) \cdot \frac{F_{tot}}{A_{mem}}}$

where J_(O2) is the oxygen flux per membrane area, X_(O2) is the molefraction of O₂ in the exiting He sweep stream, X_(N2) is the molefraction of N₂ in the exiting He sweep stream, F_(tot) is the total flowrate of gas exiting the low P_(O2) compartment of the oxygen permeationcell, and A_(mem) is the active area of the membrane. During the firstpart of the experiment, the oxygen flux was determined at severaltemperatures between 880° C. and 1050° C.

During the second part of the experiment, 250 ml/min of a sweep gasconsisting of 97.5% by volume of He and 1.25% by volume each of CO andCO₂ was flushed across the low p_(O2) surface of the membrane. Oxygenpermeated through the membrane from the high p_(O2) side to the lowp_(O2) side and combined with CO on the low P_(O2) side to form CO₂. Theconcentrations of O₂, N₂, CO and CO₂ in the exiting gas stream wereanalyzed by gas chromatography. The oxygen flux was calculated by theformula:$J_{O_{2}} = {\left\lbrack {\frac{X_{{CO}_{2}} - {\frac{X_{{CO}_{2}}^{0}}{X_{CO}^{0}} \cdot X_{CO}}}{{2 \cdot \frac{X_{{CO}_{2}}^{0}}{X_{CO}^{0}}} + 2} - X_{N_{2}}} \right\rbrack \cdot \frac{F_{tot}}{A_{mem}}}$

where J_(O2) is the oxygen flux per membrane area, X_(CO2) is the molefraction of CO₂ in the exiting He sweep stream, X_(CO) is the molefraction of CO in the exiting He sweep stream, X⁰ _(CO2) is the molefraction of CO₂ in the entering He sweep stream, X⁰ _(co) is the molefraction of CO in the entering He sweep stream, X_(N2) is the molefraction of N₂ in the exiting He sweep stream, F is the total flow rateof the gas stream exiting the low P_(O2) compartment of the oxygenpermeation cell, and A_(mem) is the active area of the membrane. Duringthe second part of the experiment, the oxygen flux was determined atseveral temperatures between 880° C. and 1050° C.

During the third part of the experiment, a pure He stream of 250 ml/minwas flushed across the low P_(O2) side of the membrane, and the oxygenflux was determined in the same manner as during the first part of theexperiment. The oxygen flux was determined at several temperatures.

FIG. 3 shows the oxygen flux (left abscissa, fully drawn line) and thetemperature (right abscissa, dashed line) as function of time during theoxygen permeation test. The first part of the test takes place in theperiod from 4 hours to 26 hours, the second part from 26 hours to 53hours, and the third part from 53 hours to 70 hours. The oxygen fluxdoes not vary substantially with time at constant temperature. Theoxygen flux during the second part of the test calculated for 1000° C.and 1 mm membrane thickness was 0.30 ml/(cm²min).

EXAMPLE 12 (COMPARATIVE) Oxygen Permeation Test of a Dense MixedConducting La_(0.4)Ca_(0.6)Fe_(0.5)Ti_(0.5)O_(3−d) Membrane

An oxygen permeation test was conducted according to the proceduredescribed in Example 11, except the membrane disk of Example 2 was used.

FIG. 4 shows the oxygen flux (left abscissa, fully drawn line) and thetemperature (right abscissa, dashed line) as function of time during theoxygen permeation test. The first part of the test takes place in theperiod from 4 hours to 24 hours, the second part from 24 hours to 118hours, and the third part from 118 hours to 135 hours. The oxygen fluxdecreases with time at a constant temperature during the first part ofthe test. During the second part of the test, there is first an apparentincrease in the oxygen flux, then a decrease. The oxygen flux during thequasi-steady state period of the second part of the test calculated for1000° C. and 1 mm membrane thickness was 0.21 ml/(cm²min).

EXAMPLE 13 Structure of La_(0.4)Ca_(0.6)Fe_(0.485)Ti_(0.485)O_(3−d)After Oxygen Flux Testing

The membrane of Example 1, tested for oxygen flux in Example 11, wasexamined by X-ray diffraction on both sides. FIG. 5 shows X-raydiffractograms of the material prior to the experiment (bottom) and ofthe two surfaces of the membrane after the oxygen permeation test; thehigh p_(O2) surface (middle) and the low p_(O2) surface (top). The peakslabelled “Si” belong to silicon which was added to the sample as aninternal standard. The peaks labelled “Al” belong to the aluminiumsample holder. The sample possesses the perovskite structure and issingle phase. It shows no evidence of decomposition after the oxygenflux experiment.

EXAMPLE 14 Structure of La_(0.55)Ca_(0.45)Fe_(0.63)Ti_(0.34)O_(3−d)After Oxygen Flux Testing

The material prepared in Example 4 was examined by Scanning ElectronMicroscopy after an oxygen flux test. Representative pictures of thehigh p_(O2) side and the low p_(O2) side are shown in FIG. 6 and FIG. 7,respectively. The micro-structure is fine-grained and homogeneous withno apparent difference between the two sides of the membrane.Semi-quantitative elemental analysis by EDS shows that the compositionof the material is essentially unchanged from before the oxygen fluxtest.

EXAMPLE 15 (COMPARATIVE) Structure ofLa_(0.4)Ca_(0.6)Fe_(0.5)Ti_(0.5)O_(3−d) After Oxygen Flux Testing

The membrane of Example 2 was examined by X-ray diffraction and ScanningElectron Microscopy after the oxygen flux test of Example 12.

FIG. 8 shows X-ray diffractograms of the material prior to theexperiment (bottom) and of the two surfaces of the membrane after theoxygen permeation test; the high p_(O2) surface (middle) and the lowp_(O2) surface (top). The peaks labelled “Al” belong to the aluminiumsample holder. Before the experiment, the sample possesses theperovskite structure and is single phase. The X-ray diffractogram of thehigh p_(O2) surface after the oxygen flux tests shows the presence ofadditional phases, of which one was identified as CaFe₂O₄. Peakslabelled “CF” in the X-ray diffractogram belong to this phase. Peakslabelled “U” belong to an unidentified phase. Unlabelled peaks belong tothe perovskite phase. The X-ray diffractogram of the low p_(O2) sideafter the oxygen permeation test shows essentially no change from thesample before the test.

FIG. 9 and FIG. 10 show representative Scanning Electron Micrographs ofthe high p_(O2) and low p_(O2) surfaces of the membrane after the oxygenflux test, respectively. FIG. 9 shows that the high p_(O2) surface ofthe membrane is covered by a continuous layer of decomposition phases.Semiquantitative elemental analysis by EDS indicates that this layerconsists of CaFe₂O₄ and an iron oxide. FIG. 10 shows that the low p_(O2)surface is fine grained and homogeneous. Semi-quantitative elementalanalysis by EDS indicates that the composition is essentially unchangedfrom before the experiment.

EXAMPLE 16 Structure of La_(0.3)Ca_(0.7)Fe_(0.485)Ti_(0.485)O_(3−d)After Oxygen Flux Testing

The membrane of Example 5 was examined by X-ray diffraction and ScanningElectron Microscopy after an oxygen flux test.

FIG. 11 shows X-ray diffractograms of the material prior to theexperiment (bottom) and of the two surfaces of the membrane after theoxygen permeation test; the high p_(O2) surface (middle) and the lowp_(O2) surface (top). The peaks labelled “Al” belong to the aluminiumsample holder. Before the experiment, the sample possesses theperovskite structure and is single phase. After the experiment, anadditional minor peak appears at d=2.7 Å. This peak is attributed to anordered structure similar to the known phases LaCa₂Fe₃O₈ and Ca₃Ti₂FeO₈.

FIG. 12 and FIG. 13 show representative Scanning Electron Micrographs ofthe high p_(O2) and low p_(O2) surfaces of the membrane after the oxygenflux test, respectively. The microstructure is essentially the same onboth surfaces. The matrix phase consists of rounded grains up to about 1micron in size. Semiquantitative elemental analysis by EDS indicatesthat this phase is essentially identical to the material before theoxygen flux test. A secondary phase consisting of elongated grains of upto 3 microns in length and less than 0.5 micron in width is also foundon both surfaces of the membrane. Semi-quantitative elemental analysisby EDS indicates that this phase has the molar ratio (La+Ca):(Fe+Ti) ofclose to unity, characteristic of the perovskites. The phase is enrichedin Ca and Fe and depleted in La and Ti relative to the bulk of thematerial. This is consistent with the formation of a phase with anordered structure similar to the known phases LaCa₂Fe3O₈ and Ca₃Ti₂FeO₈.

EXAMPLE 17 Thermal and Chemical Expansion ofLa_(0.45)Ca_(0.55)Fe_(0.63)Ti_(0.34)O_(3−d)

A solid mixed conducting membrane was prepared according to the methodof Example 4 except the membrane disk was ground into a square ofapproximately 8×8 mm. This specimen was placed in a dilatometer, andheated at a rate of 6° C./min in a flowing air atmosphere to 997° C. Theaverage thermal expansion coefficient measured between 400° C. and 997°C. was 11.8*10⁻⁶ K⁻¹. The sample was maintained at 997° C. for severalhours, whereupon the atmosphere was changed to a flowing mixture of 95%N₂, 1% CO and 4% CO₂. The sample was allowed to expand to itsequilibrium length. The atmosphere was then changed back to flowing air,whereupon the sample, still maintained at 997° C. was allowed tocontract to its equilibrium length. The relative difference in length isreferred to as the chemical expansion, and was 0.06%.

EXAMPLE 18 (COMPARATIVE) Thermal and Chemical Expansion ofLa_(0.8)Ca_(0.2)FeO_(3−d)

The membrane of Example 6 was tested by the procedure of Example 17,except the temperature of the measurement was 1005° C. The averagethermal expansion coefficient measured between 400° C. and 1000° C. was11.1*10⁻⁶ K⁻¹. The chemical expansion was 0.15%.

EXAMPLE 19 (COMPARATIVE) Thermal and Chemical Expansion ofLa_(0.6)Ca_(0.4)Fe_(0.777)Ti_(0.194)O_(3−d)

The membrane of Example 7 was tested by the procedure of Example 17,except the temperature of the measurement was 994° C. The averagethermal expansion coefficient measured between 400° C. and 1000° C. was12.3*10⁻⁶ K⁻¹. The chemical expansion was 0.12%.

EXAMPLE 20 (COMPARATIVE) Thermal and Chemical Expansion ofLa_(0.2)Ca_(0.3)Mn_(0.4)Ti_(0.6)O_(3−d)

The membrane of Example 8 was tested by the procedure of Example 17,except the temperature of the measurement was 1000° C. The averagethermal expansion coefficient measured between 400° C. and 1000° C. was11.6*10⁻⁶ K⁻¹. The chemical expansion was 0.38%.

EXAMPLE 21 (COMPARATIVE) Thermal and Chemical Expansion ofLa_(0.65)Ca_(0.35)Fe_(0.63)Ti_(0.24)Al_(0.10)O_(3−d)

The membrane of Example 9 was tested by the procedure of Example 17,except the temperature of the measurement was 995° C. The averagethermal expansion coefficient measured between 400° C. and 990° C. was11.1*10⁻⁶ K⁻¹. The chemical expansion was less than 0.01%.

EXAMPLE 22 (COMPARATIVE) Thermal and Chemical Expansion ofLa_(0.15)Sr_(0.85)Fe_(0.8)Cr_(0.10)Co_(0.10)O_(3−d)

The membrane of Example 3 was tested by the procedure of Example 17,except the temperature of the measurement was 996° C. The averagethermal expansion coefficient measured between 400° C. and 990° C. was17.0*10⁻⁶ K⁻¹. The chemical expansion was 0.27%.

EXAMPLE 23 Oxygen Permeation Test of aLa_(0.63)Ca_(0.37)Fe_(0.63)Ti_(0.34)O_(3−d) Membrane

An oxygen permeation test is conducted according to the proceduredescribed in Example 11, except a membrane disk of a compositionrepresented by the formula La_(0.63)Ca_(0.37)Fe_(0.63)Ti_(0.34)O_(3−d),is used. FIG. 14 shows a ternary diagram of the systemLaFe_(w)O_(3−d″)CaTi_(w)O_(3−d″)CaFe_(w)O_(3−d″). The composition islocated near the LaFe_(w)O_(3−d″)CaTi_(w)O_(3−d″) join of the ternarysystem, at the point marked “A”. Compositions near theLaFe_(w)O_(3−d″)CaTi_(w)O_(3−d″) join are characterised by a lowconcentration of oxygen vacancies, especially on the high p_(O2) side ofthe membrane. Low oxygen flux is obtained with this material.

EXAMPLE 24 Oxygen Permeation Test ofLa_(0.85)Ca_(0.95)Fe_(0.145)Ti_(0.825)O_(3−d) Membrane

An oxygen permeation test is conducted according to the proceduredescribed in Example 11, except a membrane disk of a compositionrepresented by the formula La_(0.05)Ca_(0.95)Fe_(0.145)Ti_(0.825)O_(3−d)is used. FIG. 14 shows a ternary diagram of the systemLaFe_(w)O_(3−d″)CaTi_(w)O_(3−d″)CaFe_(w)O_(3−d″). The composition of thematerial is located near the CaTi_(w)O_(3−d″) apex of the ternarysystem, at the point marked “B”. Low electronic conductivity, especiallyat low oxygen partial pressures is characteristic of materials withcompositions in this region, and a low oxygen flux is measured.

EXAMPLE 25 Oxygen Permeation Test ofLa_(0.25)Ca_(0.75)Fe_(0.63)Ti_(0.34)O_(3−d) Membrane

An oxygen permeation test is conducted according to the proceduredescribed in Example 11, except a membrane disk of a compositionrepresented by the formula La_(0.25)Ca_(0.75)Fe_(0.63)Ti_(0.34)O_(3−d)is used. FIG. 14 shows a ternary diagram of the systemLaFe_(w)O_(3−d″)CaTi_(w)O_(3−d″)CaFe_(w)O_(3−d″). The composition of thematerial is located near the centre of the ternary system, at the pointmarked “C”. Ordering of oxygen vacancies and cations is characteristicof materials within the system with compositions with CaFe_(w)O_(3−d″)content higher than a limit depending on the temperature and oxygenpartial pressure. The obtained oxygen flux decreases with time as theordered phase forms.

These Examples demonstrate that the oxygen separation membranes of thepresent invention are particularly suitable as membrane materials inprocesses in which the membrane is subjected to a large potentialgradient, e.g. oxygen partial pressure difference of 6-7 orders ofmagnitude or more across the membrane. Compared with compositions knownin the prior art, these compositions offer improved resistance tokinetic decomposition and reduced chemical expansion, as well asimproved stability against reduction to metal and reaction with carbondioxide and water.

What is claimed is:
 1. A solid multicomponent membrane for use in areactor wherein the membrane comprises a mixed metal oxide having astructure represented by the formula:(La_(1−x)Ca_(x))_(v)(Fe_(1−y−y′)Ti_(y)Al_(y′))_(w)O_(3−d) wherein x, y,y′, w, v and d each represent a number such that 0.1≦(y+y′)≦0.8,0.15≦(x+y′)≦0.95, 0.05≦(x−y)≦0.3, 0.95≦w<1, v=1, y′>0 and d equals anumber that renders the compound charge neutral and is not less thanzero and not greater than about 0.8.
 2. The membrane according to claim1, wherein the x, y, y′, w, v and d each represent a number such that0.15<(y+y′)<0.75, 0.2<(x+y′)<0.9, 0.05<(x−y)<0.15, 0.95<w<1, and dequals a number that renders the compound charge neutral and is not lessthan zero and not greater than about 0.8.
 3. The membrane according toclaim 1, wherein 0<y<0.75 and 0<y′<0.3.
 4. In a membrane reactor forgenerating heat by oxidation of a carbon containing fuel to CO₂ and H₂Oon the oxidation side of the membrane reactor, the improvement whichcomprises employing a membrane reactor containing the membrane ofclaim
 1. 5. In a method for generating synthesis gas consisting of oneor more of the components CO, CO₂, H₂ and N₂ in a membrane reactor wherethe reactor is capable of reacting a mixture of steam and a carboncontaining fuel with oxygen permeated through said membrane to makesynthesis gas, the improvement which comprises employing a membranereactor containing the membrane of claim 1.